President Obama announced Friday that about 100 U.S. troops have been deployed to the West African country of Niger, where defense officials said they are setting up a drone base to spy on al-Qaeda fighters in the Sahara.

It was the latest step by the Pentagon to increase its intelligence-gathering across Africa in response to what officials see as a rising threat from militant groups.

In a letter to Congress, Obama said about 40 U.S. service members arrived in Niger on Wednesday, bringing the total number of troops based there to â€œapproximatelyâ€ 100. He said the troops, which are armed for self-protection, would support a French-led military operation in neighboring Mali, where al-Qaeda fighters and other militants have carved out a refuge in a remote territory the size of Texas.

The base in Niger marks the opening of another far-flung U.S. military front against al-Qaeda and its affiliates, adding to drone combat missions in Afghanistan, Yemen and Somalia. The CIA is also conducting drone airstrikes against al-Qaeda targets in Pakistan and Yemen.

Senior U.S. officials have said for months that they would not put U.S. military â€œboots on the groundâ€ in Mali, an impoverished nation that has been mired in chaos since March, when a U.S.-trained Malian army captain took power in a coup. But U.S. troops are becoming increasingly involved in the conflict from the skies and the rear echelons, where they are supporting French and African forces seeking to stabilize the region.

Obama did not explicitly reveal the drone base in his letter to Congress, but he said the U.S. troops in Niger would â€œprovide support for intelligence collectionâ€ and share the intelligence with French forces in Mali.

A U.S. defense official, speaking on the condition of anonymity to provide details about military operations, said that the 40 troops who arrived in Niger on Wednesday were almost all Air Force personnel and that their mission was to support drone flights.

The official said drone flights were â€œimminentâ€ but declined to say whether unarmed, unmanned Predator aircraft had arrived in Niger or how many would be deployed there.

The drones will be based at first in the capital, Niamey. But military officials would like to eventually move them north to the city of Agadez, which is closer to parts of Mali where al-Qaeda cells have taken root.

â€œThatâ€™s a better location for the mission, but itâ€™s not feasible at this point,â€ the official said, describing Agadez as a frontier city â€œwith logistical challenges.â€

The introduction of Predators to Niger fills a gap in U.S. military capabilities over the Sahara, most of which remains beyond the reach of its drone bases in East Africa and southern Europe.

The Pentagon also operates drones from a permanent base in Djibouti, on the Horn of Africa, and from a civilian airport in Ethi-o-pia.

The U.S. military has been flying small turboprop surveillance planes over northern Mali and West Africa for years, but the PC-12 spy aircraft have limited range and lack the sophisticated sensors that Predators carry.

U.S. military contractors have been flying PC-12 surveillance aircraft from Agadez for several months. Those planes do not carry military markings and only require a handful of people to operate.

In contrast, Predators need ground crews to launch, recover and maintain the drones. Those crews, in turn, require armed personnel for protection.

The U.S. defense official said it is likely that more U.S. troops will deploy to Niger but declined to be specific. "I think itâ€™s safe to say the number will probably grow,â€ he said.

The Predators in Niger will only conduct surveillance, not airstrikes, the official said. â€œThis is purely an intelligence-gathering mission,â€ he said. Other officials said the Obama administration had not ruled out arming the Predators with missiles in the future.

Information collected from reconnaissance missions will be shared with the French and other African militaries so they can attack al-Qaeda targets, officials said.

There is evidence that al-Qaeda fighters in West Africa are already bracing for drone warfare. The Associated Press reported finding an al-Qaeda document in Timbuktu, Mali, that listed 22 tips for avoiding drones. Among other countermeasures, it advised hiding â€œunder thick treesâ€ and buying off-the-shelf electronic scramblers â€œto confuse the frequencies used to control the drone.â€

Niger, one of the poorest countries in the world, signed an agreement with the United States last month that provides legal safeguards for U.S. forces stationed there. Nigerien officials are concerned about the spillover of violence and refugees from Mali, which has threatened to destabilize the entire region.

Because Maliâ€™s coup leaders toppled a democratically elected government, the U.S. government is prohibited by law from giving direct military aid to Mali.

Johnnie Carson, the State Departmentâ€™s top diplomat for Africa, told reporters Friday that security assistance and other aid could â€œimmediatelyâ€ resume to Mali â€œif there is a restoration of democracy.â€ Mali has tentatively scheduled elections for July.

The French military launched a surprise intervention in Mali last month after Islamist fighters swept south and threatened to take over much of the country.

Since then, about 4,000 French troops and a coalition of about 6,000 African forces have retaken major cities in northern Mali, chasing al-Qaeda fighters and other militants into remote areas. One French official described combat operations there as â€œa little like Afghanistan.â€

French military leaders have said they would begin a partial withdrawal next month. Their strategy hinges on enlisting Malian troops and other African forces to act as peacekeepers, while negotiating side deals to persuade some of Maliâ€™s many militant factions to turn against al-Qaeda.

But al-Qaeda fighters and other Islamist militants have quickly adopted guerrilla tactics and show no sign of disappearing. Car bombs and suicide attacks have flared in recent days and are likely to intensify in the coming weeks, Carson acknowledged.

â€œThereâ€™s no question that [al-Qaeda] has not been totally defeated, but they have been significantly degraded,â€ he said at a breakfast sponsored by the Center for Media and Security.